1. Field Of The Invention
The present invention relates to a thermoplastic resin polymer and a composition. More particularly, the present invention is concerned with a novel thermoplastic resin polymer of a polyphenylene ether and a low molecular weight styrene polymer bonded to the polyphenylene ether in a limited amount, which thermoplastic resin polymer is excellent in moldability, heat resistance and transparency. The present invention is also concerned with a novel thermoplastic resin polymer composition comprising the above-mentioned thermoplastic resin polymer and a styrene polymer, which composition is excellent not only in moldability and color tone, but also in physical properties, such as impact resistance, elongation and surface gloss.
2. Discussion Of Related Art
Polyphenylene ethers (hereinafter often referred to simply as "PPE") generally have excellent mechanical strengths, electrical characteristics, heat resistance and transparency. However, PPE has a drawback in that its moldability is very poor because of its high melt viscosity. Particularly, when PPE is heated, the molecular weight of PPE is likely to increase due to the coupling between PPE molecules. When PPE has been subjected to severe thermal history, it undergoes gelation so that molding of PPE can no longer be conducted. In addition, discoloration of PPE, e.g., browning occurs when PPE has been subjected to thermal history.
In order to improve the moldability of PPE, U.S. Pat. No. 3,383,435 proposed a blend of PPE and a styrene resin (hereinafter often referred to simply as "PS"). However, the invention in this U.S. patent is not directed to the modification of PPE per se but is only directed to the improvement of melt processability of PPE by blending it with a PS. However, such blends have a disadvantage in that, when PS is used in an amount sufficient for obtaining satisfactory moldability of the blend, the heat resistance and mechanical properties such as impact resistance and tensile strength of the final product decrease considerably.
U.S. Pat. No. 3,356,761 discloses that when styrene is added to PPE, the styrene serves as a plasticizer so that the resultant mixture has melt processability at a temperature below the melting point of PPE per se. However, this effect is solely ascribed to the plasticizing performance of the styrene. Accordingly, when the mixture is subjected to melt processing, the styrene is polymerized to high degree in the presence of a crosslinking agent and the like, thereby causing the moldability of the mixture to be lowered.
On the other hand, methods have been proposed in which a styrene compound is graft polymerized on PPE to obtain a graft copolymer comprising PPE and a high molecular weight PS grafted thereon (see U.S. Pat. Nos. 3,929,930 and 4,097,556). In these methods, in order to promote graft polymerization of a high molecular weight PS on PPE, the graft polymerization is conducted in the presence of a radical polymerization initiator under specific conditions with respect to the amount of styrene compound, reaction temperature and mixing. However, as a result of the graft polymerization reaction in the presence of the radical initiator under heating, not only the graft polymerization but also the coupling reaction between PPE molecules are promoted, thereby causing the molecular weight of PPE to be increased. Consequently, the moldability of the resultant graft polymer at high temperatures is not sufficiently improved as compared to that of PPE per se. At the same time, the molecular weight of PS tends to increase during the course of the graft polymerization reaction, so that the resultant graft polymer not only becomes poor in heat resistance but also undergoes discoloration. Further, when PPE is used in combination with a rubber-modified polystyrene, another problem is caused by the use of a radical polymerization initiator. That is, the rubbery polymer used as an impact modifier is likely to undergo severe gelation by the action of the radical polymerization initiator, so that the impact strength of the resultant polymer becomes poor.